The present invention relates generally to lighting systems having low total harmonic distortion characteristics, and more particularly to a lighting system including an inventive configuration of light emitting devices such as, for example, LEDs, to achieve low total harmonic distortion characteristics.
In lighting systems and technology, there has been and continues to be an ever increasing desire to achieve a number of competing and often conflicting goals. For example, these goals include, inter alia, reliability, minimal cost, and minimization of electrical interferences. This is not a complete list. In particular, the goal of minimizing electrical interferences has proven difficult to achieve without increasing costs and decreasing reliability.
Lighting systems typically connect to alternating current (AC) electrical power source and generate light by drawing current from the AC power source. In the U.S., the AC power provides a cyclical voltage of approximately 120 volts RMS (root mean square) with a peak voltage value ranging from approximately positive 170 volts (V) to approximately negative 170 volts. In Europe and other countries, the available AC power is approximately 240 volts RMS. Other countries may use a different frequency, for example, 50 Hz. Other platforms (for example, aircraft avionics) may use another frequency such as 400 Hz. The same principles apply to the following discussion regardless of applied oscillatory voltage or frequency.
The AC power is cyclical with an oscillation frequency of approximately 60 Hertz (Hz) for the example application. Each complete voltage oscillation is considered a complete power cycle and includes 360 degrees. A sample AC power cycle is often illustrated as a sinusoidal graph as illustrated in FIG. 1, which illustrates a number of oscillations of the AC power voltage as represented by a solid line graph 120v. In FIG. 1, the horizontal axis represents time flowing from left to right, and the vertical axis for solid graph 120v represents voltage amplitude in volts. As illustrated, a single power cycle, in this example, lasts approximately 16.7 milliseconds (ms) which is one second divided by 60 cycles.
Electrical interferences are often measured in total harmonic distortion (THD) compared to the input AC power. In the present context, THD is a measure of extent or magnitude to which the wave shape of the electrical current drawn from the AC power is distorted compared to the sinusoidal shape of the AC voltage 120v. In numerical terms, THD is expressed as a percentage calculated as the ratio of the sum of the powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental frequency. In the present example, the fundamental frequency of the AC power is 60 Hz. It is desirable to minimize electrical interferences generated by a lighting system by minimizing lighting system THD.
Many current lighting systems use fluorescent bulbs, especially for industrial and commercial applications. Fluorescent bulbs are more efficient compared to incandescent bulbs. However, fluorescent bulbs are notoriously noisy. That is, fluorescent bulbs draw current from the AC power source such that undesirably high levels of total harmonic distortions (THD) are generated. This is illustrated using FIGS. 1A and 2A.
FIG. 2A illustrates a lighting system including a fluorescent bulb 10 connected to an electrical plug 12. The plug 12 is adapted to engage in a socket that provides the electrical power 120, the alternating current (AC) described above. In FIG. 2A, the load on the provided AC power 120 is the fluorescent bulb 20. Often, an inductor 15 is serially connected with the bulb 10 to limit the current flowing through the bulb 10. A representative dashed graph 10i is an approximation of the shape of the current through the bulb 10. The actual conduction duration, the maximum and minimum currents +IMAX and −IMIN, and the exact shape of the representative dashed graph 10i depend on a number of factors. The factors may include, for example only, wattage rating of the bulb 10, ambient temperature, exact waveshape and characteristics of the power voltage 120v, characteristics of the inductor 15, many others not listed here, or a combination of any one or more of these factors. For the purpose of discussing the background, the exact numerical value and the exact shape of these curves are not important; however, the maximum positive and negative currents, +IMAX and −IMAX typically range between plus and minus 670 mA (peak of the AC waveform). The shape of the illustrated curve 10i is one possible sample shape only and may not indicate the exact shape of the current flow graph which may vary widely as already noted above.
FIG. 1A is a graph illustrating electrical characteristics of the lighting system of FIG. 2A. Referring to FIGS. 1A and 2A, the AC power voltage 120v is a sinusoidal shaped graph 120v having 60 Hz oscillation. Current through the fluorescent bulb 10 is represented by representative dashed graph 10i. The applied AC power 120 drives current flow (as illustrated by the representative dashed graph 10i) through the fluorescent bulb 10. As illustrated in FIG. 1A, the shape of the current 10i through the fluorescent bulb 10 is highly dissimilar to the sinusoidal shape of the AC voltage 120v. In fact, the shape of the current 10i is exceedingly distorted compared to the shape of the AC voltage 120v. This is because the fluorescent bulb 20 presents a highly non-linear load to the applied AC voltage 120v. This is caused by a number of factors including, for example only, the operating characteristics of fluorescent bulbs. The high degree of distortion of the current 10i means that the total harmonic distortion is correspondingly high.
In some implementations, the THD value of fluorescent bulbs exceeds 100 percent. That is, more current is drawn at non-fundamental frequencies compared to the current drawn at the fundamental frequency. Such high THD value leads to a number of undesired affects such as, for example, stresses to wires, circuits, and all other systems connected to the same AC source 120. Further, the high THD value results in undesired levels of electrical noise to all surrounding and commonly connected circuits and electrical systems. In some jurisdictions, there are efforts to limit and regulate the THD values of various circuits allowed to be operated within the jurisdiction.
In most fluorescent bulb based lighting systems, the fluorescent bulb is isolated from the AC power 120 by a ballast circuit that operates to reduce the THD. FIG. 2B illustrates the lighting system of 2A with a ballast 17 connected to the fluorescent bulb 10 on one side and the electrical plug 12 on the other side. The ballast 17 regulates the current flowing through the fluorescent bulb 10 to decrease distortion of the shape of the current, thereby reducing the THD. However, the ballast 17 introduces additional electrical components. These additional electrical components increase the costs and reduce the reliability of the fluorescent bulb based lighting system.
New and increasing popular lighting technology involves the use of light emitting diodes (LEDs). LEDs are cost effective and have higher luminous efficacy compared to incandescent bulbs and fluorescent bulbs. FIG. 3 illustrates a lighting system including a first light emitting diode (LED) 21 connected to the plug 12 in a first direction and a second light emitting diode (LED) 22 connected to the plug 12 in the opposite direction and also connected to the LED 21 in parallel. Collectively, the LEDs 21 and 22 are referred to herein as the LED pair 20. As with the lighting system FIG. 2A, the plug 12 is adapted to engage in a socket that provides the electrical power 120 as described above. In FIG. 3, the load to the electrical power 120 is the LED pair 20. LEDs are diodes that conduct electricity in one direction. To take advantage of the alternating current power source 120, two LEDs are configured as shown to produce light. Often, a resistor 25 is serially connected with the LED pair 20 to limit the current flowing through the LED pair 20.
FIG. 1B is a graph illustrating electrical characteristics of the lighting system of FIG. 3. Referring to FIGS. 1B and 3, during the positive portion 121 (also, the “positive swing”) of each power cycle, node 122 is at positive voltage compared to node 124. During the positive swing 121, the first LED 31 is forward biased and the second LED 32 is reverse biased, thus, no current flows through the second LED 32. However, after a threshold voltage (+VTH) is reached, current flows through the first LED 31, generating light.
During the negative portion 123 (also, the “negative swing”) of each of the power cycles, tab point 124 is at positive voltage compared to tab point 122. During the negative swing 123, the first LED 31 is reverse-biased and the second LED 32 is forward biased, thus, no current flows through the first LED 31. However, after a threshold voltage (−VTH) is reached, current flows through the second LED 33, generating light.
The lighting system of FIG. 3 has electrical characteristics similar to that of the lighting system of FIG. 2A, though possibly with a different current waveform. The representative dashed graph 16i of FIG. 1B approximates the shape of the current through the LED pair 20. The actual conduction duration, the maximum and minimum currents +IMAX and −IMIN, and the exact shape of the representative dashed graph 16i depend on a number of factors. The factors may include, for example only, wattage rating of the LED pair 20, ambient temperature, exact shape and characteristics of the power voltage 120v, characteristics of the resistor 25, many others not listed here, or a combination of any one or more of these factors. For the purpose of discussing the background, the exact numerical value and the exact shape of these curves are not important. In one example, the maximum positive and negative currents, +IMAX and −IMAX typically range between plus and minus 80 mA in either direction.
The value of the threshold voltage (positive and negative) depends on the value of the resistor 25 and characteristics of the LED pair 20. The amount of current depends on a number of factors including the wattage rating of the LEDs 20 and the value of the resistor 25. Again, for our purposes here, the exact numerical values of these are not important.
As illustrated in FIG. 1B, the shape of the current (represented by dashed line graph 16i) through the LED pair 20 is not similar to the sinusoidal shape of the AC voltage 120v and is, in fact, very distorted compared to the shape of the AC voltage 120v. This is because the LED pair 20 presents a highly non-linear load to the applied AC voltage 120v. This is caused by a number of factors including, for example only, the way LEDs operate to generate light. The high degree of distortion of the current 16i means that the total harmonic distortion is correspondingly high. In fact the THD for the LED pair is often over 100 percent.
To realize even lower THD values for LED based lighting systems, some suggested use of complex LED driver circuits between the LEDs and the power source. For example, U.S. Pat. No. 6,304,464 to Jacobs teaches the use of a complex “flyback converter” for, inter alia, THD reduction. In another example, U.S. patent application Ser. No. 11/086,955 having a filing date of Mar. 22, 2005 and publication date of Sep. 28, 2006 teaches the use of a complex “digital power converter for driving LEDS.” The use of these LED driver circuits introduces additional electrical components. These additional electrical components increase the complexity and the costs, and reduce the reliability of these LED systems.
Accordingly, the need remains for LED based lighting systems having even lower levels of THD values while eliminating or minimizing the need for additional circuits and components.